The present invention relates to improved methods of detecting the active toner concentration of a two-component developer comprising a mixture of carrier particles and toner particles including active and fatigued toner particles for use with an electrostatic copying apparatus or the like.
Conventionally, various attempts have been made to detect the active toner concentration of two-component developers. These include an optical method wherein a developer comprising carrier particles having a different color from that of toner particles is illuminated, and the mixing ratio of toner particles and carrier particles is detected by a reflected light from the developer; and a magnetic method wherein the mixing ratio of toner particles and carrier particles is detected through a change of the magnetic permeability of the developer.
Additionally, methods of measuring the density or specific gravity, or the conductance of the developer have been proposed.
However, in the optical method, the color of carrier particles has to be different from that of toner particles. Therefore, the kinds of carrier particles that can be employed in this method are limited, for example, to resin-coated carrier particles, and the development method therefor is also limited to some extent.
Moreover, the developer does not change much in color with a change of the toner concentration of the developer so that the detecting accuracy of the toner concentration is apt to be decreased.
The magnetic method is one of the most reliable methods that have been put to practical use. However, its detecting accuracy and sensitivity are not completely satisfactory.
In a method disclosed in Japanese Patent Sho-46-8280 and Japanese Laid-Open Patent Application Sho-51-19540, a developer that has been used in the development process is caused to pass through a non-magnetic cylinder around which a coil is wound and the mixing ratio of toner particles and carrier particles of the developer is detected by a change of the inductance of the coil. A somewhat similar procedure is described in U.S. Pat. No. 3,999,687.
This method utilizes the phenomenon that the magnetic permeability of the developer that passes through the non-magnetic cylinder changes in accordance with the content of carrier particles in the developer. In other words, the phenomenon that a change of the toner concentration gives rise to a change of the bulk density of the developer is utilized.
However, in the case of the developers which are usually used, the toner concentration is in the range of from 1 to 3 wt %, and the change of the bulk density is as low as approximately 7 to 8% for each 1 wt % change of toner concentration, since the specific gravity of the carrier particles is about 7 to 8 times that of the toner particles.
The difficulty with procedures such as these, which depend upon measurement of change in inductance is that such measurements are not capable of distinguishing between fatigued and active toners. The reason for this is that inductance depends upon the presence of magnetic carrier particles, not upon the presence of toner, and as pointed out above, there is very little change in the content of the carrier particles in the developer with change in toner concentration. Moreover, procedures which depend solely upon inductance measurements to determine toner concentration are not capable of distinguishing between fatigued and active toner particles since the inductance value determined is dependent upon the change in carrier concentration, and the toner concentration is determined as the difference between the initial toner and carrier concentration and the change in carrier concentration as measured by change in inductance.